For the mammoth ivory carvings in natural ivory color, we must use the angles of schreger lines to distinguish them from elephant ivory. Please find the photos of the cross section of mammoth ivory and elephant ivory as below:

You can clearly see that the angles of the schreger lines of mammoth ivory is <90 degree while the angle of elephant ivory is >115 degree. To conclude, angles of less than 90 degree indicate that the ivory is mammoth and angles greater than 115 degree showing that it is elephant ivory.